1. Field of the Invention
The present invention relates to semiconductor devices, and in particular, a depletion type transistor having an insulated gate and its application to a solid state image pickup device.
2. Description of the Related Art
The choice of transistors for semiconductor device fabrication depends in part on the size of a particular transistor, as well as on the noise the transistor exhibits during its operation. For many applications, circuit designers prefer enhancement type MOS transistors over bipolar transistors due to the relatively small footprint of the MOS transistor compared with the bipolar transistor. Of the two transistor types, MOS transistors can be more densely integrated on a single semiconductor device than can bipolar transistors.
One typical use of enhancement type MOS transistors is as a switching element in various types of digital devices such as semiconductor memories. Another application of enhancement type MOS transistors is for analog control over the amount of transistor current flow, as for example through adjacent bipolar transistors. However, since enhancement type MOS transistors are formed with a channel near the surface of their silicon substrate, they are likely to generate more noise than bipolar transistors. Consequently, enhancement type MOS transistors exhibit a reduced signal-to-noise ratio (S/N ratio) when compared with bipolar transistors. To avoid this noise problem, circuit designers have begun to use depletion type MOS transistors for analog current control.
A depletion type MOS transistor is characterized by an inversion layer formed in the channel between its source and drain. For example, with an N type impurity like phosphorus or arsenic doped in the channel between the source and drain of an N channel MOS transistor, an N type inversion layer would be formed in the channel. This type of transistor is known as a buried channel type or depletion type N channel MOS transistor (hereinafter called DpNMOS transistor). In a DpNMOS transistor, current flows at a level deeper than the surface of the channel. This reduces the signal noise (i.e., increases the S/N ratio), making the DpNMOS an ideal choice for analog transistor current control. Alternatively, if a P type impurity such as boron were to be doped in the channel of a P channel MOS transistor, a P type inversion layer would be formed in the channel, making the device a depletion type PMOS transistor.
Generally, DpNMOS transistors function in response to a negative voltage applied to their gates. Consequently, using a DpNMOS transistor to replace an enhancement type NMOS transistor would require a circuit that converts the positive voltage, applied at the gate of the enhancement type NMOS transistor, into a negative voltage. Such a circuit would, moreover, have to serve as a negative power supply for the DpNMOS transistor. These requirements, unfortunately, complicate and enlarge the overall structure of integrated circuitry.